Colon cancer is very difficult to treat once it has spread beyond the primary site. Because of the poor results with systemic chemotherapy, it is essential to find other forms of therapy. Radioimmunotherapy (RIT) is an active field of research. The availability of several antibodies (Abs) to colon cancer-associated antigens makes this tumor a potential candidate for RIT. Little is known about the pattern of Ab penetration into tumors and the resulting heterogeneous dose distributions and their biological effectiveness. The Specific Aims of this project are: 1. To evaluate the kinetics of Ab delivery, penetration and uptake into tumors. 2. To elucidate the dosimetry of RIT at the cellular level and to create models to allow dose prediction, 3. To investigate the effect of different Abs and isotopes in defined experimental models. Four human colon cancer cell lines will be used in this project, grown as multicellular spheroids and as xenografts in nude mice. The spheroid model comprises a heterogeneous population of cells representative of micrometastases and tumor micro- environments in vivo. Results in spheroids can then guide in vivo experiments. Several monoclonal Abs to carcino-embryonic antigen (CEA) and their fragments will be used. Specific Aim 1 will be addressed by incubating spheroids for varying times with I-125- labelled Ab. Penetration will be assessed by autoradiography and immunofluorescence. Similar experiments will be performed in nude mice. Specific Aim 2 is being addressed by the construction of two mathematical dosimetry models, one for micrometastases, the second for vascularized tumors. The effect of overlapping dose distributions from multiple areas of isotope deposition and the effects of varying concentrations of isotope in the tumor will be analyzed with these models. The model will be evaluated for several beta- emitters of potential use in RIT. Miniature thermoluminescent dosimeters will be used to determine actual absorbed dose in both spheroids and xenografts. Correlations will be made with Ab distribution as determined by autoradiography and comparisons will be made with the predicted dose from the models. For Specific Aim 3. RIT experiments will be performed in spheroids and xenografts using isotopes and Abs (intact vs. fragments) that are predicted to be useful by earlier studies. Growth delay and clonogenic survival assays will be used as end points. At the completion of this project, planned directions will be: 1. the evaluation of tumors other than colon cancer, 2. research on combined modality therapy and 3. the development of clinical trials.